Overview

Spinal manipulation has long been used to treat patients with
low back and neck pain. The most common form of this
manipulation has been termed high-velocity low-amplitude,
meaning the thrust is performed at a high-velocity but with little
depth at the skin contact on the patient. Clinical skills in
applying such a manipulation lies in the chiropractor’s ability
to control speed, length and force of the load as well as the
direction in which the load is applied, and the contact point
on which the load is applied. Control over its mechanical
delivery is therefore then assumed to be related to its clinical
effects. Biomechanical changes evoked by a manipulation are
thought to have physiological consequences caused, at least in
part, by changes in neural output from paraspinal tissues.

If the activation of neural pathways does contribute to the
effects of a manipulation, it seems reasonable to anticipate
that neural discharge might increase or decrease as the thrust
duration approaches a threshold value. We hypothesized that
the relationship between the duration of an HVLA thrust to
a vertebrae and paraspinal muscle spindle discharge would be
nonlinear. In addition, we anticipated that muscle spindle discharge
would be more sensitive to larger amplitude thrusts.

Thus, we conducted a neurophysiological study of spinal
manipulation using the lumbar spine of an animal model. We
gave impulse thrusts to the spinous process of the L6 vertebra
of deeply anesthetized animal models while recording nerve
activity from the dorsal roots of muscle spindle afferent nerves
innervating the lumbar paraspinal muscles. We used a feedback
motor to deliver the impulse thrusts. The motor’s drive arm was
securely attached to the L6 spinous process via a forceps.

We found that as thrust duration became shorter, the
discharge of the lumbar paraspinal muscle spindles increased
in a particular fashion (curvilinear). A concave-up inflection
occurred near the 100-ms duration, eliciting both a higher
frequency discharge compared with the longer durations and
a faster rate of change as thrust duration was shortened. This
pattern was seen quite clearly in paraspinal afferent nerves
with receptive fields both close and far from the midline.
Paradoxically, spindle afferents were almost twice as sensitive
to the 1-mm thrust compared with the 2-mm-amplitude
thrust. This may be related to the small versus large signal
range properties of muscle spindles.

We concluded that the results indicate that the duration and
amplitude of a spinal manipulation elicit a pattern of discharge
from paraspinal muscle spindles different from slower mechanical
inputs. Clinically, this may be an important factor in the
therapeutics benefit of manipulation.

The full abstract of this study was published in The Spine Journal,
Volume 7, Issue 5, September-October 2007, pages 583-595.

The Palmer Center for Chiropractic Research
is located on Palmer’s Davenport Campus in
the William and Jo Harris Building.

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